A Short-Period Seismic Sensor
Jim Lehman

lehmancj at shentel dot net

 

 

ONE OF THE STANDARDS FOR THE PROFESSIONAL WWSSS (WORLD-WIDE STANDARD SEISMOGRAPH NETWORK) IS THE l-SECOND VERTICAL SENSOR. THE l-SEC SEISMIC DESIGN IS THE REAL WORKHORSE OF PROFESSIONAL SEISMOLOGY IN THE AREA OF RESEARCH. GREATER DEFINITION OF ARRIVING WAVEFRONTS - DIRECT, REFLECTED & REFRACTED ARE POSSIBLE.

 

THERE FOLLOWS SOME GENERAL INFORMATION ON THE DESIGN & CONSTRUCTION OF A l-SEC  VERTICAL SENSOR. PLEASE NOTE: THIS IS A SUGGESTED DESIGN FROM ECONOMICAL MATERIALS. MODIFICATIONS & IMPROVEMENTS ARE ALWAYS IN STYLE WHEN WORKING WITH SENSOR DESIGN. NOTE: UNLIKE THE TRADITIONAL LONG-PERIOD INSTRUMENT, THE SHORT-PERIOD SENSOR NEEDS X10 THE RECORDING SPEED-THAT IS 10-12 FT/HR (3.3-3.8 M/HR). THIS DEMANDS EI1HER A DRUM RECORDER OR INTERFACING WI1H A COMPUTER/PRINTER COMBINATION.

 

 

THE SKETCH BELOW SHOWS THE SALIENT PARTS OF A l-SEC VERTICAL DES IGN. A SPRING IS ANCHORED TO A FRAME. SUSPENDED FROM THE SPRING IS A COPPER DAMPING PLATE AND THE PICKUP MAGNET. THESE THREE ITEMS FORM THE INSTRUMENTAL PENDULUM THAT MUST HAVE A FREE SWING PERIOD OF l-SEC. DAMPING MAGNETS ARE ADDED ADJACENT TO THE COPPER PLATE TO DAMPEN OUT THE NATURAL PERIOD. THE PICKUP COIL IS PLACED ON IHE FLOOR WITHIN IHE PICKUP MAGNET. AS THE VERTICAL MOTION OF A SEISMIC EVENT REACHES THE SENSOR, THE COIL MOVES UP & DOWN IN RELATIVE TO THE ALMOST MOTIONLESS INSTRUMENTAL PENDULUM, AND AN OUTPUT RESULTS. BY USING AVAILABLE MATERIALS AND MODIFYING THE. DESIGN AS NECESSARY, ONE CAN BUILD THIS SENSOR QUITE ECONOMICALLY.

 

 

THE SENSOR PHOTO SHOWN ON PAGE 1 WAS MADE FROM COMPONENTS MANY OF WHICH ARE FOUND IN A "HARDWARE" STORE OR AROUND THE SHOP AREA. THE BASE IS A 1 1/2 IN (4.0 CM) THICK SECTION OF SEASONED WOOD, APPROXIMATELY 6.0 IN BY 12 IN, (15.0 X 30.0 CM). TWO 3/4TH IN PIPE FLANGES ARE BOLTED TO THE BASE WITH CENTERS 7.0-8.0 IN (20 .0 CM) APART. ONE-HALF INCH COPPER TUBING (HARD WALL) FORMS THE UPRIGHT SUPPORTS .3/4TH IN PIPE TO 1/2 IN COPPER ADAPTERS ARE USED WITH THE FLANGES. COPPER 90-DEGREE ELBOWS ARE USED AT 1HE TOP AND ALL APPROPRIATE JUNCTIONS ARE SOLDERED. THE DISTANCE FROM THE BASE TO THE TOP OF COPPER CROSS-PIECE (IN OUR CASE) IS 22.0 IN, (55.0 CM).

 

THE OVERALL HEIGHT DEPENDS ON THE SPRING SELECTED & THE SPACING NECESSARY BE1WEEN THE DAMPING MAGNETS & THE PICKUP MAGNET. THE SENSOR, (PAGE 1 PHOTO) HAS A SPRING WITH A l-SEC PERIOD WHEN 670 GRAMS IS ATTACHED. THE PICKUP MAGNET WEIGHS ABOUT 165 GM. THE 1/4 IN THICK DAMPING COPPER PLATE MADE UP THE BALANCE OF THE MASS --(APPROX. 4.0 IN (10 .0 CM) SQUARE). SEVERAL INCHES OF 3/4 IN WOODEN DOWEL SEPARATE THE PICKUP MAGNET FROM THE DAMPING MAGNET ZONE. COMPARED TO THE HORIZONTAL SENSOR DESIGN, THE RESTORING FORCE OF IHE VERTICAL PENDULUM IS MUCH LARGER AND A VERY STRONG MAGNETIC FIELD IS NECESSARY TO ACHIEVE THE PROPER DAMPING RATIO. EVEN THOUGH THE WOODEN DOWEL ADDS HEIGHT TO THE DESIGN, THE ATTRACTION OR REPULSION BETWEEN THE STRONG DAMPING MAGNETS AND THE WEAKER PICKUP MAGNET MUST BE MINIMIZED BY ADEQUATE SPACING. (ANY HARDWARE USED IN THE INSTRUMENTAL PENDULUM MUST BE NON-MAGNETIC, I .E. BRASS)

 

THE SUSPENDED SPRING IS ATTACHED TO THE UPPER CROSSPIECE BY A SCREW/NUT ARRANGEMENT THAT PERMITS EASY VERTICAL ADJUSTMENT OF AN I NCH OR MORE .THE PICKUP COIL FORM IS DESIGNED TO FIT WITHIN THE PICKUP MAGNET WITH SUFFICIENT CLEARANCE ON THE TOP & SIDES. THE COIL IS WOUND WITH 5000 TO 8000 TURNS OF #32 OR #34 MAGNET WIRE AND MOUNTED TO A SMALL BASE THAT RESTS ON THE SEISMIC BASE. WIRE LEADS ARE DIRECTED OUT FROM THE COIL TO THE AMPLIFIER.

 

THE DAMPING MAGNETS ARE ATTACHED AS SHOWN IN THE PHOTO, PAGE 1. THE SCREW/NUT ARRANGEMENT MAKE FOR EASY ADJUSTMENT TOO & FROM THE DAMPING PLATE.

 

THE OVERALL HEIGHT OF THE SENSOR SUPPORT WI LL DEPEND ON THE LENGTH OF THE INSTRUMENTAL PENDULUM, IE SPRING, DAMPING PLATE, WOODEN DOWEL AND PICKUP MAGNET. IT IS BEST TO HANG EVERYTHING FROM A TEMPORARY SUPPORT ABOVE THE COIL ON THE BASE TO GET AN APPROXIMATION OF HEIGHT NECESSARY. THERE MAY BE SOME FATIGUE IN THE SPRING AND THE TOP SUPPORT ADJUSTMENT SHOULD COMPENSATE FOR THIS. THE INSTRUMENTAL PENDULUM SHOULD HAVE A I-SEC NATURAL SWING PERIOD AND BE ALIGNED TO MATCH THE ADJACENT COMPONENTS -- DRAWING PLATE TO DAMPING MAGNETS AND PICKUP MAGNET TO PICKUP COIL.

 

LEAVE THE SUSPENDED PENDULUM EXTENDED FOR SEVERAL DAYS. SOME FATIGUE MAY APPEAR, BUT STABILITY IN LENGTH SHOULD BE OBVIOUS WITH NO FURTHER CHANGE IN LENGTH. THE DAMPING MAGNETS CAN BE MOUNTED PARALLEL TO THE DAMPING PLATE. AS THE PENDULUM IS DISPLACED FROM EQUILIBRIUM -- WITH PROPER DAMPING -- IT SHOULD RETURN AND OVERSHOOT ABOUT 1/5TH TO 1/8TH OF THE ORIGINAL DISPLACEMENT .

 

I N SUMMARY, THE COMPLETED SYSTEM SHOULD:
HAVE A 1-SEC SWING.
HAVE PROPER ALIGNMENT OF ITS PARTS.
PROVIDE PROPER CLEARANCE OF COMPONENTS.
HAVE A DAMPING RATIO OF 6:1 - 8:1.
SHOW STRUCTURAL STABILITY FOR WEEKS AT A TIME.
BE COVERED FROM AIR DRAFTS, ETC. A PLEXIGLAS COVER IS IDEAL
TO EASILY SEE WITHIN --- A CARDBOARD BOX WILL DO.

SUGGESTED SOURCES OF COMPONENTS--SHORT PERIOD SEISMIC SENSOR

SPRING:

AVAILABLE TRUE VALUE HARDWARE STORES,
SERVALITE PRODUCTS , SPRING #59,
"K" VALUE ABOUT 27 GM/CM

COPPER PLATE/DAMPING:

AVAILABLE -- SMALL PARTS INC., MIAMI LAKES FL
#A-SMC-250 1/4 IN THICK, 3 IN X 6 IN.

DAMPING MAGNETS:

AVAILABLE TRUE VALUE HARDWARE STORES
#07207, MASTER MAGNETICS 100 LB HOLDING MAGNETS.

PICKUP MAGNET / HORSESHOE:

AVAILABLE SELECTED HARDWARE STORES / EDMUND SCIENTIFIC,
6 OZ 25 LB PULL IDEAL -- SOLD AS GENERAL ALNICO
MAGNETS, EDMUND SCIENTIFIC S41948.

COIL: 

MUST BE CUSTOM WOUND TO FIT MAGNET SELECTED --
#32, 34, OR 36 MAGNET WIRE -- l/4 LB SPOOL SHOULD D0,
AVAILABLE SARGENT WELCH SCIENTIFIC SUPPLY--RADIO SHACK
PERHAPS, MOTOR / TRANSFORMER REPAIR SHOP

SPECIAL NOTE ON MAGNETS -- THERE IS WITHIN MOST MICROWAVE OVENS
A FINE PAIR OF CIRCULAR DOUGHNUT MAGNETS OF GOOD QUALITY.
THIS TAKES SOME DIGGING, BUT WI1H DOZENS OF MICROWAVE
OVENS TOSSED, THIS IS A RELATIVELY GOOD SOURCE
OF MAGNETS THAT CAN BE MOUNTED EASILY WITH EPOXY.
(Editors note: be sure to discharge the large capacitor within the microwave by
shorting out the terminals with the blade of a screw driver that has a well insulated
handle.)

lehmancj at shentel dot net

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